Typical electrical power distribution arrangements in residences include a load center having a main circuit breaker and a plurality of branch circuit breakers. The main circuit breaker serves to protect the wiring which provides electrical power from the power company into the load center, and the branch circuit breakers serve to protect the wiring extending from the load center and makes up each of the individual circuits within the residence. Typically, the main circuit breaker and branch circuit breakers provide protection for two types of faults: overload conditions and short circuit conditions.
Additionally, branch circuit breakers are available which provide ground fault protection for the individual circuits. These types of branch circuit breakers are mandated by the National Electric Code for circuits which extend into portions of the residence such as bathrooms, basements and garages, and are intended to protect personnel. To provide this protection, these branch circuit breakers are required to provide detection of ground faults (e.g., a difference in current between the neutral line and power line) above 5 milliamps within a specified time period.
While the main circuit breaker and branch circuit breakers without ground fault protection protect for overload and most short circuit conditions, these devices cannot prevent all short circuit situations. More specifically, the main and branch circuit breakers will not detect as a short circuit current, a current which is lower than its respective instantaneous tripping current or overload tripping current. Additionally, lowering an instantaneous tripping level will cause problems such as nuisance tripping, which is common when an electric motor or other device having a large inductance is coupled to the circuit.
Since the main and branch circuit breakers of a load center cannot protect against all possible short circuit conditions, damage to electrical wiring in a home which results in a short circuit between the power conductor and the neutral or ground can cause excess heating within the wiring at the point of the short circuit. As a result, this heating can further destroy the insulation around the wires and, in some situations, start a fire at the point of the short circuit. This type of damage is known to occur both in the permanent wiring of a home, as well as temporary wiring such as extension cords.
It has been found that electrical wiring which includes a ground conductor in addition to the neutral conductor, and one or more circuit conductors, will usually produce a line to ground fault when the wiring is damaged. Accordingly, protecting wiring with a circuit breaker including ground fault detection results in a significant reduction in the amount of time a wiring damaging event exists (e.g., excess heating caused by short circuit condition). More specifically, the ground fault detection feature of a circuit breaker will detect a short circuit when there is current leakage to ground.
One way of providing ground fault detection to all of the circuits extending from a load center, is to substitute branch circuit breakers including ground fault detection for all of the conventional branch circuit breakers in the load center. The main problem with this solution is cost. More specifically, a typical branch circuit breaker having the ground fault interrupt feature costs between 5 and 10 times as much as the branch circuit breaker it replaces. By way of example, a typical 100 amp load center may have the ability to hold 20 branch circuit breakers which cost in the range of $5-$10 for a total cost of between $100-$200. If branch circuit breakers having a ground fault detection feature are substituted, the cost of the circuit breakers could conceivably jump to $2,000 or more. (The cash values referred to above are estimates based upon list prices.) While this alternative is offered for consumers and electricians, they typically make their own decision to forego circuit breakers including ground fault detection due to the added cost.
In addition to costing more, providing ground fault detection at a level which provides protection for personnel (e.g., 5 milliamps of current to ground) is not required to adequately protect against low level short circuit conditions. More specifically, protecting against ground faults in the range of 300 milliamps should be adequate to protect against wiring damage caused by low level short circuits.
Accordingly, the need exists for ground fault detection which will detect a level of ground fault current which may be higher than that required for personnel protection, for all of the circuits extending from a load center which would greatly reduce damage to wiring caused by a short circuit condition which destroys the wiring insulation and causes a current flow between the power conductors, neutral and ground at the location of damage.